A controller controls first and second switching elements so that, in both of the waveforms of alternating currents flowing through the first and second heating elements, a first period including both of a phase control waveform in which a current flows in a part of a half cycle of alternately current and a wave-number control waveform in which a current flows or does not flow over a half cycle of alternately current and a second period including only the wave-number control waveform alternately appear in a control cycle; when the first heating element operates in the first/second period, the second heating element operates in the second/first period; and both the waveforms of the alternating currents flowing through the first and second heating elements are electrically symmetric in the positive and negative directions during the control cycle.